PROJECT SUMMARY/ABSTRACT Approximately one-third of the world's population is latently infected with TB (LTBI) and have a 10% lifetime risk of developing clinical pulmonary TB (PTB). Global efforts to combat TB are hampered by the emergence of drug-resistant strains of Mycobacterium tuberculosis (Mtb), and variable efficacy of the currently licensed vaccine, M.bovis BCG (BCG). After pulmonary infection with Mtb, aerosolized bacteria are inhaled and interact with the host, resulting in the recruitment of immune cells to the lung to form the tubercle granuloma. Although the presence of granuloma has long been considered a hallmark of TB, the immunological differences between a protective granuloma and a non-protective granuloma has been elusive. Our recent published data, suggest that the presence of inducible Bronchus Associated Lymphoid Tissue (iBALT) within granulomas is indicative of protective granulomas that mediate Mtb control. In contrast, infiltrating neutrophils are characteristic of granulomatous inflammation in PTB patients. These new findings significantly change the overall consensus that TB granulomas in general are protective, but instead put forth the new paradigm that during TB, protective granulomas contain iBALT, while non-protective granulomas are neutrophilic. Our new preliminary data demonstrate that Group 3 Innate Lymphoid cells (ILC3) are among the first innate cells to rapidly accumulate in the lungs upon Mtb infection, and localize within B cell follicles in iBALT-containing granulomas. ILC3 deficiency in mice also results in increased early Mtb susceptibility, and coincides with reduced macrophage accumulation and poorly formed iBALT structures. Thus, the work proposed in this grant will mechanistically address a functional innate role for ILC3 in Mtb infection. In Specific Aim 1 we will define the host factors that mediate early lung ILC3 accumulation following Mtb infection. In Specific Aim 2, we will determine the mechanism(s) via which ILC3 mediate formation of iBALT-containing granulomas and facilitate Mtb control. In Specific Aim 3, we will determine a role for ILC3 in vaccine-induced immunity against TB, and identify new ways to target ILC3 to improve immunity against Mtb infection. Together these aims will provide novel evidence for a critical role for ILC3 in mediating iBALT formation and inducing protective host immunity to TB. Identifying new ways to target ILC3 cells to improve vaccine-immunity as proposed here, will open up novel avenues that can be harnessed for TB vaccine design.